Schiff Base Ligands Derived from l-Histidine Methyl Ester: Characterization, Racemization, and Dimerization of Their Transition-Metal Complexes

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Abstract

Reactions of methyl (S)-2-[(2-hydroxybenzylidene)amino]-3-(1H-imidazol-4-yl)propanoate (S-H2LMe) and various transition-metal(II) salts (MX2: M = Mn, Fe, Co, and Ni; X = ClO4, Cl, OAc, etc.) are examined. During the reactions with MnII and CoII salts, the MII ion is oxidized by air to form a cationic complex with two mono-deprotonated ligands, [MIII(S-HLMe)2]X [M, X = Mn, ClO4 (S-1ClO4); Co, ClO4 (S-3ClO4); and Co, Cl (S-3Cl)], where (S-HLMe) is coordinated to a MIII center in a tridentate κ3O,N,N′ mode with a mer configuration. In contrast, the NiII ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [NiII(S-HLMe)(S-H2LMe)]PF6 (S-6PF6), which forms a dimer structure by double hydrogen bonds between the coordinating phenoxide and phenol groups in the crystal. Interestingly, the above complexes show slow racemization of the ligand, as confirmed by circular dichroism spectroscopy. Racemic NiII complexes of [Ni(HLMe)(H2LMe)](PF6 or Cl) (rac-6PF6 or rac-6Cl) are isolated and their crystal structures are confirmed by X-ray diffraction analysis. In the case where Co(OAc)2 is used as the metal(II) salt, a new ligand dimerization reaction takes place to afford a complex of [Co(H2LLMe)]Cl (7Cl, H4LLMe = methyl (E)-3-{[3-(1H-imidazol-4-yl)-1-methoxy-1-oxopropano-2-yl]amino}-2-[(1H-imidazol-4-yl)methyl]-2-[(2-hydroxybenzylidene)amino]-3-(2-hydroxyphenyl)propanoate).

Original languageEnglish
Pages (from-to)1733-1742
Number of pages10
JournalEuropean Journal of Inorganic Chemistry
Volume2018
Issue number16
DOIs
Publication statusPublished - Apr 30 2018

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Dimerization
Schiff Bases
Coordination Complexes
Transition metals
Ligands
Salts
Propionates
Ions
Circular dichroism spectroscopy
Phenol
Dimers
X ray diffraction analysis
Hydrogen bonds
Crystal structure
Metals
Crystals
histidine methyl ester
Air
perchlorate

Keywords

  • Dimerization
  • Histidine
  • Imidazole
  • Racemization
  • Schiff bases
  • Transition metals

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

@article{b34ce00efa7f4b4896ae3507d9738575,
title = "Schiff Base Ligands Derived from l-Histidine Methyl Ester: Characterization, Racemization, and Dimerization of Their Transition-Metal Complexes",
abstract = "Reactions of methyl (S)-2-[(2-hydroxybenzylidene)amino]-3-(1H-imidazol-4-yl)propanoate (S-H2LMe) and various transition-metal(II) salts (MX2: M = Mn, Fe, Co, and Ni; X = ClO4, Cl, OAc, etc.) are examined. During the reactions with MnII and CoII salts, the MII ion is oxidized by air to form a cationic complex with two mono-deprotonated ligands, [MIII(S-HLMe)2]X [M, X = Mn, ClO4 (S-1ClO4); Co, ClO4 (S-3ClO4); and Co, Cl (S-3Cl)], where (S-HLMe)– is coordinated to a MIII center in a tridentate κ3O,N,N′ mode with a mer configuration. In contrast, the NiII ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [NiII(S-HLMe)(S-H2LMe)]PF6 (S-6PF6), which forms a dimer structure by double hydrogen bonds between the coordinating phenoxide and phenol groups in the crystal. Interestingly, the above complexes show slow racemization of the ligand, as confirmed by circular dichroism spectroscopy. Racemic NiII complexes of [Ni(HLMe)(H2LMe)](PF6 or Cl) (rac-6PF6 or rac-6Cl) are isolated and their crystal structures are confirmed by X-ray diffraction analysis. In the case where Co(OAc)2 is used as the metal(II) salt, a new ligand dimerization reaction takes place to afford a complex of [Co(H2LLMe)]Cl (7Cl, H4LLMe = methyl (E)-3-{[3-(1H-imidazol-4-yl)-1-methoxy-1-oxopropano-2-yl]amino}-2-[(1H-imidazol-4-yl)methyl]-2-[(2-hydroxybenzylidene)amino]-3-(2-hydroxyphenyl)propanoate).",
keywords = "Dimerization, Histidine, Imidazole, Racemization, Schiff bases, Transition metals",
author = "Rina Ogawa and Yukinari Sunatsuki and Takayoshi Suzuki",
year = "2018",
month = "4",
day = "30",
doi = "10.1002/ejic.201800137",
language = "English",
volume = "2018",
pages = "1733--1742",
journal = "Chemische Berichte",
issn = "0009-2940",
publisher = "Wiley-VCH Verlag",
number = "16",

}

TY - JOUR

T1 - Schiff Base Ligands Derived from l-Histidine Methyl Ester

T2 - Characterization, Racemization, and Dimerization of Their Transition-Metal Complexes

AU - Ogawa, Rina

AU - Sunatsuki, Yukinari

AU - Suzuki, Takayoshi

PY - 2018/4/30

Y1 - 2018/4/30

N2 - Reactions of methyl (S)-2-[(2-hydroxybenzylidene)amino]-3-(1H-imidazol-4-yl)propanoate (S-H2LMe) and various transition-metal(II) salts (MX2: M = Mn, Fe, Co, and Ni; X = ClO4, Cl, OAc, etc.) are examined. During the reactions with MnII and CoII salts, the MII ion is oxidized by air to form a cationic complex with two mono-deprotonated ligands, [MIII(S-HLMe)2]X [M, X = Mn, ClO4 (S-1ClO4); Co, ClO4 (S-3ClO4); and Co, Cl (S-3Cl)], where (S-HLMe)– is coordinated to a MIII center in a tridentate κ3O,N,N′ mode with a mer configuration. In contrast, the NiII ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [NiII(S-HLMe)(S-H2LMe)]PF6 (S-6PF6), which forms a dimer structure by double hydrogen bonds between the coordinating phenoxide and phenol groups in the crystal. Interestingly, the above complexes show slow racemization of the ligand, as confirmed by circular dichroism spectroscopy. Racemic NiII complexes of [Ni(HLMe)(H2LMe)](PF6 or Cl) (rac-6PF6 or rac-6Cl) are isolated and their crystal structures are confirmed by X-ray diffraction analysis. In the case where Co(OAc)2 is used as the metal(II) salt, a new ligand dimerization reaction takes place to afford a complex of [Co(H2LLMe)]Cl (7Cl, H4LLMe = methyl (E)-3-{[3-(1H-imidazol-4-yl)-1-methoxy-1-oxopropano-2-yl]amino}-2-[(1H-imidazol-4-yl)methyl]-2-[(2-hydroxybenzylidene)amino]-3-(2-hydroxyphenyl)propanoate).

AB - Reactions of methyl (S)-2-[(2-hydroxybenzylidene)amino]-3-(1H-imidazol-4-yl)propanoate (S-H2LMe) and various transition-metal(II) salts (MX2: M = Mn, Fe, Co, and Ni; X = ClO4, Cl, OAc, etc.) are examined. During the reactions with MnII and CoII salts, the MII ion is oxidized by air to form a cationic complex with two mono-deprotonated ligands, [MIII(S-HLMe)2]X [M, X = Mn, ClO4 (S-1ClO4); Co, ClO4 (S-3ClO4); and Co, Cl (S-3Cl)], where (S-HLMe)– is coordinated to a MIII center in a tridentate κ3O,N,N′ mode with a mer configuration. In contrast, the NiII ion is not oxidized; only one of the ligands is deprotonated to form a similar monocationic complex, [NiII(S-HLMe)(S-H2LMe)]PF6 (S-6PF6), which forms a dimer structure by double hydrogen bonds between the coordinating phenoxide and phenol groups in the crystal. Interestingly, the above complexes show slow racemization of the ligand, as confirmed by circular dichroism spectroscopy. Racemic NiII complexes of [Ni(HLMe)(H2LMe)](PF6 or Cl) (rac-6PF6 or rac-6Cl) are isolated and their crystal structures are confirmed by X-ray diffraction analysis. In the case where Co(OAc)2 is used as the metal(II) salt, a new ligand dimerization reaction takes place to afford a complex of [Co(H2LLMe)]Cl (7Cl, H4LLMe = methyl (E)-3-{[3-(1H-imidazol-4-yl)-1-methoxy-1-oxopropano-2-yl]amino}-2-[(1H-imidazol-4-yl)methyl]-2-[(2-hydroxybenzylidene)amino]-3-(2-hydroxyphenyl)propanoate).

KW - Dimerization

KW - Histidine

KW - Imidazole

KW - Racemization

KW - Schiff bases

KW - Transition metals

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U2 - 10.1002/ejic.201800137

DO - 10.1002/ejic.201800137

M3 - Article

AN - SCOPUS:85046478616

VL - 2018

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EP - 1742

JO - Chemische Berichte

JF - Chemische Berichte

SN - 0009-2940

IS - 16

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